The present invention relates to a split-gate flash memory cell and its flash memory array and, more particularly, to a self-aligned split-gate flash memory cell having a single-side dish-shaped floating-gate structure and its contactless flash memory arrays.
A flash memory cell structure can be basically divided into two categories: a stack-gate structure and a split-gate structure, in which the gate length of a stack-gate structure can be defined by using a minimum-feature-size (F) of technology used and, therefore, the stack-gate structure is often used in existing high-density flash memory system for mass storage applications. The stack-gate flash memory cells can be configured into an array of a matrix form according to the basic logic function, such as NOR-type, NAND-type or AND-type. In general, a stack-gate flash memory cell in a NOR-type or AND-type array is programmed by the channel hot-electron injection (CHEI), however the programming efficiency is low and the programming power is large. Moreover, the gate length of a stack-gate flash memory cell is difficult to be scaled down due to the punch-through effect if the channel hot-electron injection is used as a programming method. For a NAND-type array, the stack-gate flash memory cells are interconnected in series with common-source/drain diffusion regions, the density is high but the read speed is relatively slow as compared to that of a NOR-type array. Moreover, the programming speed of a NAND-type array is relatively slow due to the Fowler-Nordheim tunneling across the thin tunneling-oxide layer between the source/drain diffusion region and the floating gate being used as a programming method.
The split-gate structure having a select-gate region and a stack-gate region offers in general a larger cell size as compared to that of a stack-gate structure and is usually configured to be a NOR-type array. Two typical split-gate flash memory cell structures are shown in FIG. 1A and FIG. 1B. FIG. 1A shows a split-gate flash memory device having the floating-gate layer 111 formed by a local-oxidation of silicon (LOCOS) technique, in which the floating-gate length is defined in general to be larger than a minimum-feature-size of technology used due to the bird""s beak formation; the control-gate 115 is formed over a LOCOS-oxide layer 112 and a thicker select-gate oxide layer 114; a poly-oxide layer 113 is formed over a sidewall of the floating-gate layer 111; a source diffusion region 116 and a drain diffusion region 117 are formed in a semiconductor substrate 100 in a self-aligned manner; and a thin gate-oxide layer 110 is formed under the floating-gate layer 111. The split-gate structure shown in FIG. 1A is programmed by mid-channel hot-electron injection, the programming efficiency is high and the programming power is low as compared to the channel hot-electron injection used by the stack-gate structure. Moreover, the over-erase problem of the split-gate structure can be prevented due to a high threshold-voltage for the select-gate region, so the control logic circuits for erasing and verification can be simplified. However, there are several drawbacks for FIG. 1A: the cell size is larger due to the non self-aligned control-gate structure; the gate length can""t be easily scaled down due to the misalignment of the control-gate with respect to the floating-gate; the coupling ratio is low and higher applied control-gate voltage is required for back erase; and the field-emission tip of the floating-gate layer is difficult to be controlled due to the weak masking ability of the bird-beak oxide.
FIG. 1B shows another split-gate structure, in which the floating-gate layer 121 is defined by a minimum-feature-size (F) of technology used; a thin tunneling-oxide layer 120 is formed under the floating-gate layer 121; a select-gate dielectric layer 122 is formed over the select-gate region and the exposed floating-gate layer 121; a control-gate layer 123 is formed over the select-gate dielectric layer 122; and a source diffusion region 124 and a double-diffused drain region 125, 126 are formed in a semiconductor substrate 100. From FIG. 1B, it is clearly visualized that similar drawbacks as listed for FIG. 1A are appeared except that the erasing site is located at the thin tunneling-oxide layer 120 between the floating-gate layer 121 and the double-diffused drain region 125,126.
It is therefore an objective of the present invention to provide a self-aligned split-gate flash memory cell having a cell size being smaller than 4F2.
It is another objective of the present invention to provide a higher coupling ratio for a self-aligned split-gate flash memory cell.
It is a further objective of the present invention to provide a reproducible tip-cathode structure for the self-aligned split-gate flash memory cell with a higher field-emission efficiency.
It is yet another objective of the present invention to provide two contactless architectures for self-aligned split-gate flash memory arrays.
Other objectives and advantages of the present invention will be more apparent from the following description.
A self-aligned split-gate flash memory cell of the present invention is formed on a semiconductor substrate of a first conductivity type having an active region isolated by two parallel shallow-trench-isolation (STI) regions, wherein each of the two parallel STI regions is filled with a first raised field-oxide layer. A cell region can be divided into three regions: a common-source region, a gate region, and a common-drain region, wherein the gate region is located between the common-source region and the common-drain region. The common-source region comprises a first sidewall dielectric spacer being formed over a sidewall of the gate region and on a portion of a first flat bed being formed by a common-source diffusion region of a second conductivity type in the active region and two etched first raised field-oxide layers in the two parallel STI regions. The common-drain region comprises a second sidewall dielectric spacer being formed over another sidewall of the gate region and on a portion of a second flat bed being alternately formed by a common-drain diffusion region of the second conductivity type in the active region and two etched second raised field-oxide layers in the two parallel STI regions. The gate region comprises a single-side dish-shaped floating-gate structure being formed on a first gate-dielectric layer with a first intergate-dielectric layer being formed on its top portion and a second intergate-dielectric layer being formed on its inner sidewall and tip portion; and a planarized control/select-gate conductive layer being at least formed over a second gate-dielectric layer and the first/second intergate-dielectric layers in the active region and a portion of the first/second raised field-oxide layers in the two parallel STI regions, wherein the single-side dish-shaped floating-gate structure is preferably formed by an isotropic dry or wet etching process.
The self-aligned split-gate flash memory cell of the present invention as described is used to implement two contactless array architectures: a NOR-type flash memory array and a parallel common-source/drain conductive bit-lines flash memory array. The contactless NOR-type flash memory array comprises a plurality of common-source conductive bus lines being formed alternately in a first direction; a plurality of common-drain conductive islands being at least formed over the plurality of active regions along each of the common-drain regions between the plurality of common-source conductive bus lines; a plurality of self-aligned split-gate flash memory cells being formed between each of the plurality of common-source conductive bus lines and its nearby common-drain conductive islands with the elongated planarized control/select-gate conductive layer being acted as a word line in the first direction; and a plurality of bit-lines integrated with the plurality of common-drain conductive islands being simultaneously patterned and etched in a second direction being perpendicular to the first direction.
The contactless parallel common-source/drain conductive bit-lines flash memory array of the present invention comprises a plurality of common-source conductive bit lines and a plurality of common-drain conductive bit lines being formed alternately in a first direction; a plurality of self-aligned split-gate flash memory cells being formed between each of the plurality of common-source conductive bit lines and each of the plurality of common-drain conductive bit lines; and a plurality of word lines integrated with a plurality of planarized control/select-gate conductive islands being simultaneously patterned and etched in a second direction being perpendicular to the first direction.